1. Field of the Invention
This invention relates generally to a method for controlling the peak luminance level of a display panel, and more specifically to a display panel driving method, a display apparatus, a display panel driving apparatus and an electronic apparatus.
2. Description of the Related Art
In recent years, development of display apparatus of the self-luminous type wherein organic EL (Electro Luminescence) devices are arranged in a matrix has been and is advancing. A display panel which uses an organic EL device is simple and easy in reduction in weight and film thickness and besides is high in response speed, and therefore is superior in a moving picture display characteristic. A display panel which uses an organic EL device is hereinafter referred to also as organic EL panel.
Incidentally, as a driving method for an organic EL panel, a passive matrix driving method and an active matrix driving method are available. Recently, development of a display panel of the active matrix driving type wherein an active device in the form of a thin film transistor and a capacitor are disposed for each pixel circuit is being carried out energetically.
FIG. 1 shows an example of a configuration of an organic EL panel having a variation function of a light emitting period. Referring to FIG. 1, the organic EL panel 1 includes a pixel array section 3, a first scanning line driving section 5 for writing a signal voltage, a second scanning line driving section 7 for controlling the light emitting period, and a data line driving section 9. Pixel circuits 11 are arranged in M rows×N columns in the pixel array section 3. The values of M and N depend upon the display resolution.
It is to be noted that a scanning line VSCAN1 in FIG. 1 is a wiring line for providing a writing timing of a signal voltage. Meanwhile, another scanning line VSCAN2 is a wiring line for providing a start timing and an end timing of a light emitting period. Further, a signal line Vsig is a wiring line for providing a signal voltage corresponding to pixel data.
FIG. 2 shows an example of a configuration of a pixel circuit 11 having a variation function of the light emitting period. It is to be noted that various circuit configurations have been proposed for such pixel circuits. FIG. 2 shows a one of comparatively simple ones of such circuit configurations.
Referring to FIG. 2, the pixel circuit 11 shown includes a write control device T1, a current driving device T2, a light emitting period control device T3, a holding capacitor Cs and an organic EL device OLED.
In the pixel circuit 11 shown in FIG. 2, an N-channel thin film transistor is used for the write control device T1 and a P-channel thin film transistor is used for the current driving device T2 while an N-channel thin film transistor is used for the light emitting period control device T3.
Here, the operation state of the write control device T1 is controlled by the first scanning line VSCAN1 connected to the gate electrode of the write control device T1. When the write control device T1 is in an on state, a signal voltage corresponding to pixel data is written into the holding capacitor Cs through the signal line Vsig.
The signal voltage after written is held in the holding capacitor Cs for a period of time of one field. The signal voltage held in the holding capacitor Cs corresponds to the gate-source voltage Vgs of the current driving device T2.
Accordingly, drain current Ids having a magnitude corresponding to the magnitude of the signal voltage held in the holding capacitor Cs flows to the current driving device T2. As the drain current Ids increases, the current flowing to the organic EL device OLED increases and the emitted light luminance increases.
It is to be noted, however, that supplying and stopping of the drain current Ids to the organic EL device OLED are controlled by the light emitting period control device T3. In particular, the organic EL device OLED emits light only within a period within which the light emitting period control device T3 is in an on state. The operation state of the light emitting period control device T3 is controlled by the second scanning line VSCAN2.
Also a pixel circuit having a circuit configuration shown in FIG. 3 is used for the pixel circuit 11 having a variation function of the light emitting period. Referring to FIG. 3, the pixel circuit 11 shown is generally formed such that the voltage of a power supply line to which the current driving device T2 is connected is variably controlled to control supplying and stopping of the drain current Ids to the organic EL device OLED. The pixel circuit 11 includes a write control device T1, a current driving device T2, a holding capacitor Cs and an organic EL device OLED.
In the pixel circuit 11 shown in FIG. 3, a power supply line to which the source electrode of the current driving device T2 is connected corresponds to the second scanning line VSCAN2. To the second scanning line VSCAN2, a power supply voltage VDD of a high potential or a power supply voltage VSS2 of a low potential lower than a further power supply voltage VDD is supplied. Within a period within which the power supply voltage VDD of the high potential is supplied, the organic EL device OLED emits light, but within another period within which the power supply voltage VSS2 of the low potential is supplied, the organic EL device OLED emits no light.
FIGS. 4 and 5 illustrate relationships between voltages applied to the first scanning line VSCAN1 and the second scanning line VSCAN2 and the driving state of the corresponding pixel. It is to be noted that FIG. 4 illustrates the relationship where the light emitting period is long, and FIG. 5 illustrates the relationship where the light emitting period is short.
Incidentally, FIGS. 4 and 5 illustrate the relationships between the applied voltage and the driving state corresponding to the pixel circuits 11 from the first to third rows of the pixel array section 3. In particular, a numerical value in parentheses represents a corresponding row position.
As seen in FIGS. 4 and 5, a period within which both of the first scanning line VSCAN1 and the second scanning line VSCAN2 have the L level corresponds to a no-light emitting period.
On the other hand, a period within which the first scanning line VSCAN1 has the H level and the second scanning line VSCAN2 has the L level corresponds to a writing period of the signal voltage.
Further, a period within which the first scanning line VSCAN1 has the L level and the second scanning line VSCAN2 has the H level corresponds to a light emitting period.
The reason why a variation function of the light emitting period is incorporated in the pixel circuit 11 in this manner is that such several advantages as described below are achieved.
One of the advantages is that, even if the amplitude of an input signal is not varied, the peak luminance level can be adjusted. FIG. 6 illustrates a relationship between the light emitting period length occupying in a one-field period and the peak luminance level.
As a result, where the input signal is a digital signal, it is possible to adjust the peak luminance level without reducing the gradation number of the signal. On the other hand, where the input signal is an analog signal, since the signal amplitude does not decrease, the noise immunity can be raised. In this manner, variation control of the light emitting period length is effective to implement a pixel circuit which provides high picture quality and can easily adjust the peak luminance.
Further, the variation control of the light emitting period length has an advantage that, where the pixel circuit is of the current writing type, the writing current value can be increased to reduce the writing time.
Furthermore, the variation control of the light emitting period length is advantageous in that it improves the picture quality of moving pictures. It is to be noted that, in FIGS. 7 to 9, the axis of abscissa indicates the position on the screen and the axis of ordinate indicates the elapsed time. All of FIGS. 7 to 9 illustrate a movement of a line of sight where an emission line moves within the screen.
FIG. 7 indicates a display characteristic of the hold type display wherein the light emitting period is given as 100% of a one-field period. A representative one of display apparatus of the type just described is a liquid crystal display apparatus.
FIG. 8 illustrates a display characteristic of the impulse type display apparatus wherein the light emitting period is sufficiently short with respect to a one-field period. A representative one of display apparatus of the type described is a CRT (Cathode Ray Tube) display apparatus.
FIG. 9 illustrates a display characteristic of the hold type display apparatus wherein the light emitting period is limited to 50% of a one-field period.
As can be recognized from comparison of FIGS. 7 to 9, where the light emitting period is 100% of a one-field period as seen in FIG. 7, a phenomenon that the display width looks wider upon movement of a bright spot, that is, a motion artifact, is likely to be perceived.
On the other hand, where the light emitting period is sufficiently shorter than a one-field period as seen in FIG. 8, the display width remains short also upon movement of a bright point. In other words, a motion artifact is not perceived.
Where the light emitting period is 50% of a one-field period as seen in FIG. 9, also upon movement of a bright point, increase of the display width can be suppressed, and motion artifact can be reduced as much.
Generally, it is known that, in the case of moving pictures wherein a one-field period is given by 60 Hz, if the light emitting period is set to 75% or more of a one-field period, then the moving picture characteristic is deteriorated significantly. Thus, it is estimated that preferably the light emitting period is suppressed to less than 50% of a one-field period.
FIGS. 10 and 11 illustrate examples of a driving timing of the second scanning line VSCAN2 where a one-field period includes a single light emitting period. In particular, FIG. 10 illustrates an example of a driving timing where the light emitting period within a one-field period is 50% while FIG. 11 illustrates another example of a driving timing where the light emitting period within a one-field period is 20%. In FIGS. 10 and 11, it is illustrated that the phase relationship makes one cycle with 20 lines.
It is to be noted that the light emitting period corresponding to the sth scanning line VSCAN2(s) can be given by an expression given below. However, it is assumed that a one-field period is given by m horizontal scanning periods, and writing operation into the sth scanning line VSCAN2(s) is carried out within the sth horizontal scanning period and light emission is carried out simultaneously. Further, the ratio of the light emitting period occupying in a one-field period T is represented by DUTY.
At this time, the light emitting period and the no-light emitting period are individually given by the following expressions:
Light emitting period:[(s−1)/m]·T<t<{[(s−1)/m]+DUTY}·T 
No-light emitting period:{[(s−1)/m]+DUTY}·T<t<{[(s−1)/m]+1}·T 
where t satisfies a period given by the following expression:[(s−1)/m]·T<t<{[(s−1)/m]+1}·T 
Relating techniques are disclosed in JP-A-2002-514320, Japanese Patent Laid-Open No. 2005-027028 and Japanese Patent Laid-Open No. 2006-215213.